• the Earth's atmosphere has changed significantly since the Earth was first formed

2) Evolutionary Atmosphere - gases derrived from the earth's interior - outgassing; "purple sulfur bacteria" produced organic materials from inorganic elements (3.6 billion yrs ago)

3) Living Atmosphere - first photosynthesis by "blue-green algae" (3.3 billion yrs ago)

4) Modern Atmosphere - abundance of life as a consequence of high levels of oxygen

B. Earth's Present Day Atmosphere

Compostition & Structure:

(1) Composition of the Atmosphere

Defn - Atmosphere - a thin envelope of gases that surrounds the earth. It is held to the earth by the force of gravity, and it moves with the earth as the earth rotates.

Total mass of the atmosphere = 5.1 EE 21 grams

Total mass of the earth = 6.0 EE 27 grams

Mass of the atmosphere ~ one millionth mass of the earth

Natural Air = clean air + pollutants

Clean Air = dry air + water vapor

Gas % by Volume

Dry Air: nitrogen (N2) 78.08%

oxygen (O2) 20.95%

argon (Ar) 0.93%

He, Ne, Kr, Xe, Rn, H2 trace

Water Vapor - a variable constituent of clean air

• ~ 4% in warm, moist tropical air

• ~ 0% in cold, dry,polar air

ozone (O3), carbon monoxide (CO), sulfur oxides (SOx), nitrogen oxides (NOx, ), carbon dioxide (CO2)

(2). Vertical Structure of the Atmosphere

Pressure (weight of the overlying atmos.):

• pressure (force per unit area) of the atmosphere decreases w/height above the ground (see Fig. 14.4)
• pressure is often used as a vertical coordinate in meteorological applications

Density - [mass per unit volume (m/v)] of the atmosphere decreases with height above the ground.

ht. above the ground fraction of atmos below

5.5 km (3.4 mi)                 1/2

11 km (6.8 mi)                 3/4

16.5 km (10.2 mi)             9/10

32 km (19.9 mi)             99/100

Temperature - the atmos. can be classified by layers based on the average vertical temperature profile. (See Fig. 14.6.)

Troposphere:

• the layer of the atmosphere nearest the earth to ~ 10 km) where temperature generally decreases with height.

• the location of most of the atmosphere's moisture and consequently weather.

• upper boundary- tropopause

• the layer above the troposphere (~ 10-50 km) where temperature generally increases w/height

• very stable (warmer air over cooler air) > thus vertical motions are weak

• has a low moisture content > few clouds

• upper boundary is the stratopause

• most of the atmosphere's ozone (O3) is found here (@ ~ 25 km)

• ozone in the stratosphere is destroyed when it absorbs ultraviolet radiation from the sun (O3 molecule becomes atomic (O) and molecular (O2) oxygen)

• some of the absorbed energy warms the stratosphere the ozone layer protects biological life below from dangerous UV radiation
• top of the stratosphere is the stratopause

• the layer above the stratosphere (~ 50-80 km) where temperature decreases w/height

• upper boundary is the mesopause

• the layer above the mesosphere (above ~ 80 km) where temperature increases w/height again

• O2 (molecular oxygen) is photodissociated here

• O2 absorbs UV radiation from the sun, and in the process, heating of the atmosphere occurs

• because of the low density of molecules and atoms, a small amount of UV energy absorption produces a large temperature increase

• "homosphere" (0-80 km)- a well mixed region where the composition is uniform (78% N2, 21% O2, etc.) due to turbulent mixing

• "heterosphere"(80 km - gases are layered by atomic weight - less mixing here

Vertical Structure By Function:

• "ionosphere" (mainly in the thermosphere) - an electrified region within the upper atmosphere where large concentrations of ions and free electrons exist - it filters harmful wavelengths of solar radiaton